Optical semiconductor devices that use LEDs or the like as a light source have conventionally been in wide use as a light source for a variety of forms of display, illumination, and backlighting.
Recently, conventional structures of optical semiconductor devices include: (i) a structure generated by providing a lead frame on for example, a substrate, and then mounting a light emitting element on the lead frame, subsequently sealing the light source and its surrounding area with a sealing resin to prevent deterioration of the light source and its surrounding area due to heat, humidity, oxidation, etc.; (ii) a structure generated by mounting a light emitting element on a ceramic compact having metal wiring in a concavity, subsequently sealing the light source and its surrounding area with a sealing resin to prevent deterioration of the light source and its surrounding area due to heat, humidity, oxidation, etc.; and (iii) a structure generated by mounting a light emitting element on a hermetically sealed component composed of a metal base, lead wire, and sealing glass, and subsequently sealing the light source and its surrounding area with a sealing resin to prevent deterioration of the light source and its surrounding area due to heat, humidity, oxidation, etc.
The material for the sealing resin is required to be extremely transparent and to be able to maintain high brightness of the light source.
In recent years, optical semiconductor devices developed for uses such as illumination include white-light emitting devices that emit white light as a combination of three colors of light, or that use a blue or violet-blue light emitting element and are sealed with sealing resin that includes fluorescent material. There is an increasing need for devices that emit white light and have high output, and the demand has grown for devices that emit light at short wavelengths and that are endowed with transparency and resistance to deterioration.
In all cases, silicone resin that has superior retention properties in the areas of heat resistance and transparency as compared to epoxy resin is used as the sealing resin (for example, see Non-Patent Literature 1).
In order to obtain superior light source characteristics, it is important to increase luminous efficiency of the light source while also efficiently using light emitted from the light source. Accordingly, technology exists to cover the lead frame placed around the light source in the optical semiconductor device with a plating layer having a superb degree of reflection. Ag, a metal that has a high degree of reflection over a broad range of emission wavelengths, is widely used as the material for plating.
At present, a variety of ideas have thus been conceived to endow an optical semiconductor device with superb performance in terms of white-light emission or high emission.
In this context, optical semiconductor devices have the following problems. Upon driving an optical semiconductor device and performing an accelerated reliability test, the inventors observed that the surface of an Ag plating layer surrounding where the light emitting element is mounted and sealed with silicone resin became discolored, turning a blackish brown color. Consideration of this phenomenon revealed that, when using silicone resin that includes a resin hardening catalyst, such as metal sulfides or metal chlorides, of which platinic chloride is representative, the change in color was caused by a reaction between the catalyst and the Ag plating layer that produced Ag2S (silver sulfide), AgCl (silver chloride), or another silver halide.
The inventors also observed that the near-ultraviolet radiation produced by the light emitting element when driven accelerated the reaction producing a silver halide (silver sulfide, silver chloride, etc.); in particular, this reaction became pronounced due to blue and violet-blue light in the near-ultraviolet region. The platinic chloride or other catalyst included in the silicone resin is extremely reactive with the Ag plating layer that is in contact with the catalyst, and at the boundary surface between the silicone resin and the Ag plating layer, the Ag plating layer easily ionizes. Accordingly, the catalyst has the effect of increasing reaction to a silver halide such as silver sulfide or silver chloride. Furthermore, since silicone resin is extremely gas-permeable, it is easy for halogen gas or sulfur gas (SOx gas such as SO2) to penetrate from outside the sealing resin to the Ag plating layer. Therefore, after manufacturing, there is a risk that externally produced halogen gas or sulfur gas will come into contact with the surface of the Ag plating, triggering a reaction to produce an unwanted silver compound such as silver sulfide or silver chloride.
If the surface of the Ag plating layer near the pad on which the light emitting element is mounted turns a blackish brown color, the plating layer loses the high degree of reflection that normally characterizes Ag, and thus the degree of reflection drops conspicuously. This may cause the light yielded by the optical semiconductor device to become insufficient.
To overcome the problem of the Ag plating layer turning a blackish brown color, the inventors produced an invention with a plating laminate composed of a pure Ag plating layer and an Au—Ag alloy plating layer (Patent Literature 2). In this plating laminate, the Au—Ag alloy plating in the uppermost layer has the effect of controlling the change to a blackish brown color, while also improving the degree of reflection of light and promising an improvement in constant light source characteristics.